The invention relates to a device and a method for monitoring a beam of ionizing radiation and, more particularly, to a beam steering system including a transmission ion chamber assembly for detecting the lateral position of such a beam.
In various applications, especially in radiotherapy, it is necessary to monitor the actual beam position and to correct any deviations from the intended beam position. To do this, a number of devices--called transmission ion chambers--have been developed.
For example, Medical Physics 11 (1984), pages 105 to 128, section VI.B., discloses a multi-chamber construction of five parallel plates alternately carrying high voltage (polarizing) electrodes and ion-trapping (collecting) electrodes. Each collecting electrode is divided into four sectors such that four distinct laminar collecting volumes are designed. By summing and subtracting the current signals of specific sector pairs, the beam dose and position are measured. For correcting misalignments, the position signals are used to energize beam steering coils grouped around an electron beam.
In European patent No. 40589, there is described a modified chamber assembly which, in relevant part, varies from the above mentioned chamber in that it uses only one collector electrode split up into eight segments.
In all of these chamber arrangements, the beam position is determined by sensing the differences between signals derived from different beam areas. The chamber must therefore be struck by a large beam cross-section, i.e. placed downstream of all the beam widening (and weakening) elements. As a result, the signal and in particular the signal to noise ratio are weak and require a sophisticated signal processing system to obtain an acceptable sensitivity. Moreover, the chamber cannot be utilized in instances where the radiation field is built up by scanning rather than spreading the beam. Further, because the chamber assembly contains a multitude of electrodes and provides a plurality of signals, it is mechanically complicated and large numbers of electronic components are required to produce it.
It is an object of this invention to provide a transmission ion chamber arrangement which generates strong signals with a relatively low noise level.
It is another object of this invention to provide a versatile transmission ion chamber assembly, capable of handling different kinds of beams (for example, electron or x-ray beams) and beam diameters (for example, swept or diffused beams).
It is a further object of this invention to provide a transmission ion chamber assembly which is simple and inexpensive to produce.
It is yet another object of this invention to provide a mechanically and electronically simple system for monitoring and correcting the position of an ionizing beam.
It is still another object of this invention to provide a simple and accurate method for measuring and correcting lateral misalignments of an ionizing beam.
It is yet another object of this invention to improve upon known beam steering systems of this type.